1. Field of the Invention
This invention relates to a method of polymerizing diorganocyclosiloxane to yield polydiorganosiloxane gum having a controlled low reactivity with reinforcing silica filler.
2. Background Information
In the commercial production of heat cured silicone elastomers, a polydiorganosiloxane gum is mixed with reinforcing silica to give the final elastomer useful physical properties. The mixture of polydiorganosiloxane gum and reinforcing silica reacts to form a crepe, upon aging, if special precautions are not taken. The most common method of preventing crepe is the addition of a short chain, hydroxyl endblocked polydiorganosiloxane fluid. It is postulated that the short-chain fluid reacts with the reinforcing silica preferentially over the gum, so that the gum and reinforcing filler do not form a crepe upon aging.
Evans teaches in U.S. Pat. No. 4,122,247, issued Oct. 24, 1978, that the water content of cyclic siloxanes can be reduced by heating to 100.degree. C. or above with a nitrogen purge to an amount of less than 10 parts per million.
U.S. Pat. No. 4,439,592, issued Mar. 27, 1984, to Maass et al., discusses the preparation of triorganosilylterminated polydiorganosiloxanes. They point out that small amounts of water, in the basic polymerization, lead to Si--OH groups at the chain ends of the polymer. As a result, the properties of the polymers, for example the interaction with fillers. are very adversely effected. Their patent discloses a method of producing an improved polymer by distilling off a part of the cyclic diorganosiloxane. before polymerizing, in the presence of the polymerization catalyst at a temperature which is at least 10.degree. C. below the temperature at which polymerization begins. A dry inert gas can be passed in during the distillation and prior to it as a further drying measure.
U.S. Pat. No. 4,551,515, issued Nov. 5, 1985 to Herberg et al. teaches that their mixture of cyclopolysiloxane and chainstopping agent is preferably dried before addition of the polymerizing catalyst to eliminate water which will terminate or chainstop diorganopolysiloxane polymer with silanol groups and thus reduce the viscosity of the polymer and change the nature of it's interaction with a filler. They further teach that after the polymerization is complete and the catalyst has been neutralized, if desired, the polymer can be devolatilized to remove volatiles such as cyclopolysiloxane.